Long-Range Intramolecular Electronic Communication in a Trinuclear Ruthenium Tropolonate Complex

Dinuclear and trinuclear
ruthenium complexes, [Ru­(trop)2(C2trop)­Ru­(dppe)­Cp]
[2b; trop = tropolonato, C2trop = ethynyltropolonato,
dppe = 1,2-bis­(diphenylphosphino)­ethane] and [Ru­(trop)­{(C2trop)­Ru­(dppe)­Cp}2] (3), were synthesized,
and their electronic and electrochemical properties were investigated
in comparison with our previously reported complex [Ru­(acac)2(C2trop)­Ru­(dppe)­Cp] (2a). The electron-donating
RuII(dppe)­Cp unit and electron-accepting RuIIIO6 unit are connected by C2trop in these complexes. 2a incorporates acetylacetonate as an ancillary ligand, while 2b and 3 incorporate tropolonate as an ancillary
ligand. Every complex, 2a, 2b, and 3, exhibits similar UV–vis–near-IR (NIR) absorption
spectra, demonstrating the lack of explicit intramolecular electronic
communication between the units at least in the neutral state. The
weak NIR absorption in 2a further diminished upon electrochemical
oxidation, indicating almost no electronic communication between the
units. In contrast, 2b and 3 exhibit broad
NIR absorptions upon oxidation. Additionally, 3 exhibits
four stepwise redox couples in the electrochemical study, which are
formally attributed to [RuII(trop)3]−/[RuIII(trop)3], two [RuII(dppe)­Cp]/[RuIII(dppe)­Cp]+, and [RuIII(trop)3]/[RuIV(trop)3]+ couples. Clear
separation of the redox couples attributed to the two terminal [Ru­(dppe)­Cp]
units demonstrates the thermodynamic stability of the intermediate
oxidation states with respect to disproportionation. Further electrochemical
studies using an electrolyte including perfluorinated weakly coordinating
anions and density functional theory/time-dependent density functional
theory calculations confirmed the effect of ancillary ligands, acetylacetonate
and tropolonate. In the case of 2a, electronic delocalization
over the whole complex, especially over the [Ru­(acac)2(trop)]
unit, appears to be small. In contrast, the electronic communication
between [Ru­(dppe)­Cp] and [Ru­(trop)3] units in 3 seems to be enhanced upon oxidation, resulting in the long-range
intramolecular electronic communication.